The use of various electrochemical techniques to quantify the corrosion of a metal article or to measure the effectiveness of a given corrosion inhibition technique has been described in a number of references including the following: H. P. Hack, xe2x80x9cThe Potentiostatic Technique for Corrosion Studiesxe2x80x9d, p. 57 in Electrochemical Techniques for Corrosion Engineering, R. Baboian, ed., National Association of Corrosion Engineers, Houston 1986; J. R. Scully, Corros. Tests Standard 75-90. (R. Baboian, ed.) American Society for Testing and Materials: Philadelphia, Pa., 1995 (CAS Ref.: 125:259468); G. R. Cameron et al., Electrochemical Techniques for Corrosion Engineering, p. 183; J. Wolstenholme, Electrochemical Methods of Assessing the Corrosion of Painted Metalsxe2x80x94A Reviewxe2x80x9d, Corrosion Science 13, 521 (1973), G. W. Walter, xe2x80x9cA Critical Review of d.c. Electrochemical Tests for Painted Metalsxe2x80x9d, Corrosion Science 26, 39 (1986), and J. N. Murray, xe2x80x9cElectrochemical Test Methods for Evaluating Organic Coatings on Metals: an Update, Parts I, II, and III, Progress in Organic Coatings 30, 225 (1997), ibid. 31, 255 (1997), and ibid. 31, 375 (1997).
Such techniques divide broadly into those that interrogate the chemistry of metal and those that interrogate the quality of the organic coating. Despite the widespread use of organic or resinous coatings to protect corrosion-prone metals, such as steel and aluminum, from deterioration, it has not been standard practice to interrogate electrochemically or otherwise a metal substrate beneath an organic coating. Several recently reported methods are generally relevant to the method described herein without being suggestive thereof: electrochemical noise spectroscopy has been adapted to study underpaint corrosion (L. Meszaros et al., FATIPEC Congr., 22nd (Vol. 4), 68-71, 1994 (CAS Ref.: 124:12877)); measurement of the corrosion resistance of painted metal (M-H Khireddine, Mater. Tech. (Paris) 84, 3-8, 1996 (CAS Ref.: 125:174734)); the use of electrochemical impedance spectroscopy (EIS) for evaluation of underpaint corrosion is reported (P. L. Bonora et al., Electrochim. Acta 41, 1073, 1996; F. Mansfield, ACHxe2x80x94Models Chem., 132, 619, 1995 (CAS Ref.: 124:63152)); and EIS has also been adapted for measurement of the corrosion rates of uncoated steel alloys (A. Nishikata et al., Corros. Sci. 37, 2059, 1995 (CAS Ref.: 124:62557)). EIS is more commonly used to evaluate the integrity of paint coatings on metal substrates by analyzing the coating as an element in an electronic circuit. Reviews of the use of EIS for the evaluation of coatings can be found in the following publications: G. W. Walter, Corrosion Science 1986, 26, 681; and F. Geenan, National Technical Information Service (Order No. PB2-133479).
An electrochemical method for measurement of the barrier resistance of organic coatings (A. Metrot et al., J. Appl. Electrochem. 26, 361, 1996 (CAS Ref.: 124:273062) is also relevant to but distinct from the method described herein.
The present invention is a high throughput electrochemical test method for determining the resistance to corrosion of a metal article coated with a xe2x80x9clibraryxe2x80x9d of one or more differing coatings compositions containing candidate corrosion inhibitors. The process comprises:
(a) making, as the working electrode in an electrochemical cell which also comprises a reference electrode, a counter-electrode and an electrolytic solution, one or more metal articles comprising a plurality of coated areas thereon, with the proviso that a portion of the coating does not exist on the metal thereby allowing for the ultimate passage of electrical current to the metal without the coating being a barrier to such passage;
(b) impressing a series of direct current electrical potentials upon each of the partially coated metal articles in sequence to enable current to flow between the metal article under test in the electrochemical cell and the counter-electrode; and
(c) measuring the current flow as the direct current potential is varied relative to the reference electrode to generate a potentiodynamic scan of the active and passive regions of the metal.
It is within contemplation of the present invention that, e.g., within a series of related samples, one need measure current density at only one or several anodic bias potentials, to determine the relative merit of candidate inhibitor compositions. That is, one may achieve the desired purpose without running a complete polarization curve.
A procedure such as this where the applied potential is held constant and current is allowed to respond is characterized as a xe2x80x9cpotentiostaticxe2x80x9d procedure. If the voltage regulating potentiostat is programmed to hold a potential for some period of time, then change to and hold a new potential, the method is called xe2x80x9cpotentiodynamicxe2x80x9d. Methods in which applied voltages are increased in a positive or anodic sense are called xe2x80x9canodicxe2x80x9d scans.
In the alternative, a polarization scan can be run in a galvanostatic mode: a series of currents can be imposed and the voltage measured. However, galvanostatic methods would be expected to be inferior for the intended purpose to potentiostatic methods.
In Principles and Prevention of Corrosion, Second Edition (1996, Prentice Hall, Upper Saddle River, N.J., page 123), Denny A. Jones reviewed experimental methods for measuring active-passive metals:
. . . controlled current instrumental methods are not adequate for determining active-passive behavior, and controlled potential methods are required to show the entire anodic polarization curve.
It is apparent that galvanostatic procedures are inadequate to define the active-passive curve properly because potential is not a single-valued function of current. However, current is a single-valued function of potential, and controlled potential procedures . . . are effective in studying the electrochemical behavior of active-passive alloys.